Chapter 5 Quiz 2: sort() and limit() alternative solution

[sassbalint (Sass Bálint)]

Another solution which uses $group and 1. does not require to know that there should be some empty values and 2. gives a more detailed view of the data – which I like.

db.trips.aggregate([
    {$group:
        {_id: "$birth year", count: {$sum: 1}}
    },
    {$sort: {"_id": -1}}
])

[steevej (Steeve Juneau)]

@sassbalint

Another solution

Yes. But at a much higher processing cost. In particular if there is an index. Your solution will have to perform a collection scan and process all documents of the collection. A find/sort/limit would do an index scan and output only one document and right away when the document is found.

@sassbalint

does not require to know that there should be some empty values

Yes. But you are counting them and the result is part of your output. Having a

@sassbalint

more detailed view of the data

is interesting when it is related to your use-case.

So your solution works but is not optimal to answer the question.

[sassbalint (Sass Bálint)]

@steevej

@sassbalint

Another solution

Yes. But at a much higher processing cost.

Indeed. I was not aware of processing cost, you are right. I just think that it is worth to develop different solutions to a problem while learning and this was also a good opportunity for me to understand how $group works deeper. And maybe for others, that’s why are posted it.

@steevej

In particular if there is an index. Your solution willhave to perform a collection scan and process all documents of the collection. A find/sort/limit would do an index scan and output only one document and right away when the document is found.

Thank you for this explanation. Very useful, develops my thinking.

@steevej

@sassbalint

does not require to know that there should be some empty values

Yes. But you are counting them and the result is part of your output.

Of course. I consider this an advantage: I do not have to know anything about the data in advance, the query itself shows me the properties of the data instead.

@steevej

So your solution works but is not optimal to answer the question.

Thanks for your comment! That’s a great forum!

[steevej (Steeve Juneau)]

@sassbalint

I just think that it is worth to develop different solutions to a problem while learning

I totally agree.

Studying alternatives and starting a discussion about them is a good way to increase our knowledge.

I’m all in.